The invention relates to a method and an arrangement for controlling the rpm of a drive unit.
The course of the rpm is an important criterion especially during the start of a drive unit. The objective is to obtain a highest possible gradient when running up the motor and achieve a defined, reproducible overshoot of the rpm beyond the steady-state desired rpm for a uniform transient effect into the idle range. For this reason, measures are taken during the start phase which produce such a desired rpm behavior. This applies also to internal combustion engines wherein, as a rule, this is undertaken by controlling the air supply to predetermined start values and by controlling (open loop and/or closed loop) the rpm in accordance with the desired rpm trace by shifting the ignition angle. An example of such a procedure is provided in German patent publication 199 39 821. Because of the influencing of the ignition angle to form this rpm course, the ignition angle cannot be set to its optimal value (for example, with respect to the heating of a catalytic converter). Furthermore, such functions must also be considered in the start phase so that the flexibility for controlling the course of the rpm is limited. Additionally, the control of the rpm course is made more difficult by disturbing quantities which are superposed by the on and off switching of additional consumers such as a rear window heating, servo pump, et cetera and can considerably affect the course of the rpm.
A procedure is known from German patent publication 198 42 496 wherein an electric machine (starter generator) is mounted in the region of the drive train. With the actuation of the electric machine, an internal combustion engine can be influenced with respect to the rpm by the power output or the power take-up of the electric machine. In the known solution, this effect of the electric machine is utilized to adjust the synchronous rpm when shifting the vehicle transmission.
A torque-based control system for an internal combustion engine is presented in German patent publication 197 39 567. Here, a desired torque value is formed by coordinating different torque requests. In the above-mentioned embodiment, the desired torque value is converted into a desired torque value for the air path and a desired torque value for the more rapid crankshaft synchronous path (for example, ignition angle). The torque change requests (at least the dynamic portions of additional functions such as an idle control) are superposed on the last-mentioned desired torque value. In one embodiment, the resulting desired torque value is realized by shifting the ignition angle.
A satisfactory configuration of the rpm course is achieved by driving an electric machine (starter generator) in the start of the internal combustion engine. The electric machine influences the rpm of an internal combustion engine by taking up power and by outputting power. It is especially advantageous that air supply and ignition angle setting are adjusted uninfluenced by the rpm control exclusively in accordance with exhaust gas considerations especially of an effective heating of a catalytic converter.
The dead time of the system is reduced in an especially advantageous manner by the rapid intervention via the electric machine. In this way, the desired rpm course is better maintained which increases reproducibility and is therefore advantageous also with respect to comfort for the driver.
In an advantageous manner, the control parameters of the internal combustion engine, namely ignition and charge, are designed for a reliable starting performance of the thermal reaction heating of the catalytic converter; whereas, the electric machine takes up excessive torque in the start phase or supplies missing torque to maintain the desired rpm course.
In an especially advantageous manner, unpredictable disturbance quantities such as the on or off switching of a rear window heater or a servo pump are rapidly compensated by corresponding control of the electric machine and via its positive torque. The rear window heating or a servo pump today lead to noticeable drops in rpm.
In an advantageous manner, the request for torque by the electric machine is formed on the basis of the deviation between a pregiven desired torque and the actual torque. In this way, and in an advantageous manner, the intervention of the electric machine in the sense of increasing torque or reducing torque is considered in the torque orientated control structure of an internal combustion engine.